MDS encompasses a heterogeneous group of myeloid neoplasms characterized by dysplastic ineffective hematopoiesis, cytopenias and evolution to sAML. New technological advances allowed for improved analysis of genomic defects in leukemias, including MDS. Among new lesions, we and others have identified somatic mutations of several spliceosomal genes. This discovery has led to the proposal that alterations in the pattern of splicing of target genes play a major role in the establishment or progression of MDS and other leukemias. Molecular defects in the spliceosomal machinery open the potential for new diagnostic, prognostic and therapeutic modalities. This proposal was inspired by our recent identification of relatively common somatic mutations in DDX41 gene coding for an RNA helicase with DEAD-box domain involved in pre-mRNA splicing. The DDX41 gene, located at 5q35.3, is frequently deleted in MDS. Furthermore, we have identified multiple families with germline frameshift mutations in DDX41 that segregate with susceptibility to MDS and AML. Tumors in such patients often combine the frameshift mutation and a specific missense mutation on the other allele. Our proposal is based on the hypothesis that mutations and/or haploinsufficiency of DDX41 leads to specific types of mis-splicing of specific or distinct combinations of TSG and ultimately, that spliceosomal defects may result in pathogenetic consequences similar to those produced by direct mutations or haploinsufficient gene expression. Thus, spliceosomal mutations may phenocopy consequences of other molecular defects affecting specific genes. On the molecular level spliceosomal mutations result of change of function leading to a specific missplicing pattern of certain genes. These predictions are being confirmed in our studies of other splicing factor mutations. The partial loss of function of DDX41 seen in patient samples also suggests that a synthetic lethal approach to drug treatment might be possible in such cases. The goals of the proposal are to understand the clinical associations and pathogenic mechanisms consequent to loss of function mutations, haploinsufficient expression and missense mutations and to develop targeted therapeutic principles. The specific aims of our proposal include i) Definition of the clinical features of DDX41 lesions. ii) Understanding the role of DDX41 in pre-mRNA splicing and clarification of the effects of DDX41 mutations on helicase function, splicing and leukemogenesis using in vitro and cell culture systems. iii) Construction of mouse strains that model the genotypes observed in human patients and monitoring them for the development of bone marrow disease. In sum, our proposal, based on the discovery of DDX41 germline and somatic mutations, has the potential to elucidate a new pathogenic pathway in MDS and establish a new therapeutic target and diagnostic biomarker.